Blades of wind turbine rotors are exposed to large dynamic loads when the wind turbine is operated in turbulent wind conditions or in conditions of flow distortion, e.g. high wind shear. Until now, therefore, the blades of wind turbine rotors and the supporting structures have been dimensioned such as to be able to withstand the dynamic loads that can occur under the conditions to which the turbine is certified. In recent years it has become clear that continuing this practice of simply having sufficient structural strength for whatever loads could occur would lead to very heavy designs for large wind turbines. Consequently, various methods have been developed for the active reduction of operational loads.
One method for actively reducing the operational loads is the so-called cyclic blade pitch. In the cyclic blade pitch the pitch setting of each rotor blade is changed during each revolution, as, e.g., described in U.S. Pat. No. 4,298,313. More recently, active flap regulation has been described where aerodynamic properties of the blade profiles are changed instead of the blade pitch.
Irrespective of the chosen actuation method, any active load reduction system needs to be controlled by a regulator. A regulator for regulating the pitch angle of rotor blades is, e.g., described in EP 1 666 723 A1. A control method and a control device for controlling the pitch angles of rotor blades of a wind turbine rotor or the aerodynamic profiles of the rotor blades based on a proportional-integral control strategy (PI-control) are described in U.S. 2006/0145483 A1.
U.S. Pat. No. 4,297,076 describes a wind turbine with tip portions of the blades which are variable in pitch and device for controlling the pitch so as to relieve bending moments on the blades. The control device comprises the unit which allows for correcting the phase angle of the control signal with respect to the rotors azimuth.
EP 1 719 910 A1 describes an active vibration damping method for a wind turbine generator and device for performing the method. A damping signal component in a blade pitch angle reference signal is provide by filtering the blade pitch angle reference signal with a transfer function which includes a time constant for approximating the lag of the pitch change mechanism in responding to that blade pitch angle reference signal. A similar approach is disclosed in GB 2 117 934 A.
Bossanyi, E. A.: Individual Blade Pitch Control for Load Reduction, Wind Energy, Wiley, Chichester, GB Volume 6 (2002-10-08) pages 119 to 128 describe the possibility of using different pitch angles demands sent to each blade as a way of reducing loads. A LQG controller (linear, quadratic, gaussian controller) is used for deriving the control signals.
Caselitz P. et al.: “Reduction of Fatigue Loads on Wind Energy Converters by Advanced Control Methods”, European Wind Energy Conference, XX, XX, October 1997, pages 555 to 558 describe tilt and yaw moment compensation by individual blade control.